On account of depletion of fossil fuel reserves, as well as concerns of climate change resulting from anthropogenic activities, attention has been focused in recent years on renewable energy systems. Conventional approaches the electricity generation often involve of form of reservoir, for example a hydroelectric facility includes a water dam providing a steady stream of water which can be regulated via a water valve to drive a turbine, and a hot nuclear reactor core is capable of storing considerable thermal energy for providing a steady stream of steam under high pressure which can be regulated via a steam valve to drive a turbine. In contradistinction, many renewable energy systems driven by wind, tidal flows and ocean wave motion experience a wide range of motion amplitudes, motion velocities, motion frequencies and/or motion directions. Moreover, extreme weather conditions often result in a large dynamic range of motion at various times throughout a year. In consequence, energy pickoff from these renewable energy systems is often technically difficult to implement. A conventional approach to cope with these problems of energy pickoff is to employ hydraulic apparatus, for example hydraulic piston-cylinder pump apparatus as described in U.S. Pat. No. 6,476,511 B1 (Yemm et al.), or hydraulic pump apparatus as described in international PCT patent application WO 2007/016120A2. However, hydraulic systems are prone to wear, and are relatively energy inefficient on account of viscous drag occurring in hydraulic fluids employed therein.
As an alternative to employing hydraulic power pickoff, it is feasible to employ a variable speed generator. Several types of variable speed generators are known, but they are often very rough devices which are not really ideal for applications such as wind power or tidal energy production. Large wind turbines for electricity generation rotate with a fairly low rotation rate (rpm) and current generators designed for use with these turbines utilize either:    (i) a staged gear system between a wind turbine rotor and a generator, wherein rotational speed is controlled by adjusting wind turbine blade pitch; or    (ii) a large diameter generator directly coupled to a wind turbine rotor, wherein the generator is designed to operate at low rotation rates.
The output power from the generator is conventionally rectified and then phase formed via power electronics operating at high switching frequencies, for example at frequencies of several kHz. These approaches (i) and (ii) are relatively expensive to implement, especially in respect of electronic apparatus required to rectify and phase form the generated electrical power provided by the generator.